The present invention relates, generally to a combined analog-digital circuit for detecting the ratio of the average amplitude of an input information pulse with respect to a predetermined threshold, and more particularly, to a detector circuit which provides a digital output when the input signal to the detector exceeds a predetermined ratio of the long term average of the input level.
The analog-digital ratio detector of the present invention is designed to be used in conjunction with a band limited log receiver. The input for the detector is normally 0 to -2.0 volts and corresponds to an input signal level for the receiver system. The ratio detector can operate for either linear or log input characteristics. The properties of the log receiver are such that a constant ratio change in input level will generate a fixed output level change that is independent of the input magnitude. Therefore a ratio detector for a log receiver system detects a fixed change in input level which is independent of the magnitude of the average input level. If the ratio detector is to operate for linear input signals the threshold must become a function of the average input magnitude.
Prior ratio detectors used in this environment generally employ resistor-capacitor (RC) networks in the integration portion of the detector circuit. The inherent capacitor leakage in the RC network restricts the integration time to shorter periods and thus inhibits a precise determination of the threshold used in establishing the ratio. In addition, RC integrators increase the size requirements for the ratio detector when compared to the present invention since the RC network requires large, high quality capacitors for comparable integration times, and these capacitors add appreciably to the cost of manufacturing the detector.